GB2418479A

GB2418479A - Joule-Thompson cooling apparatus comprising two counterflow heat exchangers

Info

Publication number: GB2418479A
Application number: GB0517806A
Authority: GB
Inventors: Uwe Georg Hingst
Original assignee: Diehl BGT Defence GmbH and Co KG
Current assignee: Diehl BGT Defence GmbH and Co KG
Priority date: 2004-09-02
Filing date: 2005-09-02
Publication date: 2006-03-29
Anticipated expiration: 2025-09-02
Also published as: FR2878945B1; DE102004042398A1; FR2878945A1; DE102004042398B4; US20070000260A1; US7205533B2; GB0517806D0; GB2418479B

Abstract

The invention relates to a cooling apparatus 10 for cooling a detector 52, said apparatus comprising an inner and an outer counterflow heat exchange, 12 and 14 respectively, for a first and a second gas respectively, located in a heat-insulating housing 16, the inner counterflow heat exchanger being disposed inside a part of the length of the outer counterflow heat exchanger, and the inner counterflow heat exchanger being spatially separated from the outer counterflow heat exchanger by an outer casing 18. The outer casing then has a dividing plate 32 between an expansion nozzle 28 for the first gas, located at the end of the inner counterflow heat exchanger, and the rest of the outer counterflow heat exchanger. The rest of the outer counterflow heat exchanger, which extends beyond the inner counterflow heat exchanger, is disposed inside the outer casing. Downstream of an expansion nozzle 44 for the second gas, located at the end of the outer counterflow heat exchanger, the outer casing is closed by an end plate 46. In addition, in the region in which it is surrounded by the outer counterflow heat exchanger, the outer casing has a number of openings.

Description

24 18479.. . .. . Cooling Apparatus The invention relates to a cooling

apparatus for a detector, according to the preamble of Claim 1.

Detectors, such as for example semiconductor detectors, only achieve their optimum sensitivity to radiation at temperatures far below ambient temperature. Therefore cooling of the detectors is essential.

A cooling apparatus for cooling an object is known from EP 0 432 583 B1, said cooling apparatus being made up of two coolers connected in tandem for two different gases. With the first cooler for a first gas, it involves a counterflow heat exchanger which has an expansion nozzle located downstream of the forward flow of the second cooler for the second gas. The first gas is Repressurized at this expansion nozzle and is thus cooled. In the counterflow, the first gas of the first cooler cools both the forward flow of the second cooler for the second gas and its own forward flow. Both coolers are disposed in a heat-insulating housing. The expansion nozzle of the second cooler is located outside this housing. The cooled gas emerging there serves to cool objects located in the vicinity.

Disclosed in DE 1 501 715 is a device for liquefying gases, which can for example be used for cooling photoelectric cells. The device comprises two counterflow heat exchangers in a Dewar vessel. In this case, one counterflow heat exchanger is disposed inside the other counterflow heat exchanger. The two counterflow heat exchangers are separated from one another by an outer casing. Adjoining the inner counterflow heat exchanger, which is closed with an expansion nozzle, is a cold chamber, in which the cooled gas of the inner counterflow heat exchanger accumulates and cools its own forward flow in the counterflow. The outer counterflow heat exchanger, which is disposed around the outer casing within which the inner counterflow heat exchanger and the cold chamber are located, terminates in an expansion nozzle, in the vicinity of which the object to be cooled is situated. The gas emerging through the expansion nozzle of the outer counterflow heat exchanger cools both the object located in the vicinity of the expansion nozzle and its own forward flow in the counterflow.

2.. :.e... ce e:e.e It is disadvantageous that for certain applications, such as for example the rapid cooling of large-surface detectors, the cooling capacity of the described cooling apparatuses is not adequate.

The present invention is therefore based on the objective of producing a cooling apparatus for a detector, which has a greater cooling capacity than those of the prior art.

In respect of a cooling apparatus for cooling a detector, said apparatus comprising an inner and an outer counterflow heat exchanger for a first and a second gas respectively and being located in a heat-insulating housing, the inner counterflow heat exchanger being disposed inside part of the length of the outer counterflow heat exchanger, and the inner counterflow heat exchanger being spatially separated by an outer casing from the outer counterflow heat exchanger, said objective is achieved according to the invention in that a) the outer casing has a dividing plate disposed between an expansion nozzle for the first gas, which is located at the end of the inner counterflow heat exchanger, and the rest of the outer counterflow heat exchanger, b) the rest of the outer counterflow heat exchanger, which extends beyond the inner counterflow heat exchanger, is disposed inside the outer casing, c) downstream of an expansion nozzle for the second gas, which is located at the end of the outer counterflow heat exchanger, the outer casing is closed by an end plate, d) in the area in which it is surrounded by the outer counterflow heat exchanger, the outer casing has a plurality of openings.

The invention is based on the concept that an outer counterflow heat exchanger for a second gas undergoes thermal loading as a result of an inner counterflow heat exchanger, which is spatially separated from the outer counterflow heat exchanger by 3 a.. :.e...'t:. e:e ee.

an outer casing, at the moment when the second gas is cooled below a temperature of the first gas of the inner counterflow heat exchanger. Thus the maximum cooling capacity to be achieved is ultimately reduced.

The invention is further based on the concept that, from this moment on, the thermal loading has a more severe effect the longer the area in which the inner counterflow heat exchanger, and a cold chamber possibly disposed behind it, and the outer counterflow heat exchanger extend together along the outer casing and, by way of said casing, are subject to thermal exchange. An improvement in the cooling capacity can be achieved if the two counterflow heat exchangers only run together along a certain part of the length of the outer casing, and if the part of the outer counterflow heat exchanger which extends beyond the inner counterflow heat exchanger is disposed inside the outer casing. A further improvement in cooling capacity can be achieved by the provision of a dividing plate in the outer casing, said plate spatially separating the rest of the outer counterflow heat exchanger from the inner counterflow heat exchanger. Due to the two aforementioned measures, a certain thermal decoupling is achieved between the inner counterflow heat exchanger and the rest of the outer counterflow heat exchanger, so that the inner counterflow heat exchanger does not constitute a thermal load, or at least only constitutes a slight thermal load, in respect of the outer counterflow heat exchanger when its gas is cooled to below a temperature of the first gas of the inner counterflow heat exchanger.

The invention is further based on the concept that a great cooling capacity is achieved relatively quickly with the cooling apparatus if the outer counterflow heat exchanger is cooled not only by its own gas in the counterflow, but if in addition another gas contributes towards the cooling. Owing to the fact that the outer casing has a number of openings in the region in which it is surrounded by the outer counterflow heat exchanger, the outer counterflow heat exchanger is not only cooled by its own gas in the counterflow but also to an extent by the gas of the inner counterflow heat exchanger. The gas of the inner counterflow heat exchanger thus cools its own forward flow and, by emerging through the openings into the area in which the outer counterflow heat exchanger is located, also cools this portion of the forward flow of the outer counterflow heat exchanger. ë.

: . ::: :: 4 . .:: ..

The invention is also based on the consideration that objects to be cooled, such as for example detectors, can be damaged or adversely affected in respect of their functional capabilities - for example due to coating of the surface of the object with moisture - if they come into direct contact with the cooled or liquefied gas. Owing to the fact that the outer casing is closed by an end plate downstream of an expansion nozzle located at the end of the outer counterflow heat exchanger for the second gas, a self-contained cooling apparatus, consisting of two counterflow heat exchangers, is produced which on the one hand no longer has to compensate for external thermal loading, and on the other hand does not result in any direct contact between gas and object to be cooled.

A cooling apparatus is produced according to the invention which, when compared with the prior art, achieves more rapid cooling of a detector with comparable thermal capacity or achieves a comparable cooling time for larger detectors having higher thermal capacities.

Such cooling apparatuses are particularly suitable for use in missiles. According to the field of application, missiles or their target-detecting units must very quickly achieve full operational readiness, i.e. they must be cooled down quickly. On the other hand, it is important that, for purposes of target detection and recognition, the missile covers as large a field of view as possible. The size of the field of view covered is directly correlated with the surface of the detector used in the missile. The larger the surface of the detector which can be used depending on the cooling capacity, then the larger the field of view covered. The modern development of target-detecting units today leads to ever larger matrix detectors and thus to larger masses with corresponding thermal capacities which must be cooled down by a cooling apparatus.

Owing to the fact that the inner counterflow heat exchanger is disposed inside the outer counterflow heat exchanger, an extremely compact structure is produced. This very "slim" structure of the cooling apparatus makes it especially suitable for use in missiles, since in this case a cooling apparatus has to be accommodated in the region of the missile's seeker head, where there is only an extremely small amount of available space.

: . :: :: . : . ..

In the case of certain military applications, not only is it necessary to cool a detector to a temperature of below 100 K, but it is also necessary to reach this temperature particularly quickly. Such an extremely rapid cooling, which should involve cooling times of only one to two seconds in respect of a temperature of less than 100 K, makes high demands on a cooling apparatus.

Aside from the structure of a cooling apparatus, the cooling capacity of a cooling apparatus is also directly influenced by the two gases used for the cooling process.

For the second gas, a gas is usefully selected which, as regards its cooling capacity and boiling temperature, satisfies the applicationspecific requirements regarding the required cooling capacity and enables the minimum cooling temperature of the detector, at which a satisfactory operation of same is possible, to be achieved.

For cooling times in the region of one to two seconds and a cooling temperature of K in accordance with the aforementioned military purpose, the gases argon, nitrogen or air are especially suitable since, in all three cases, their boiling temperature is below 100 K. The first gas, however, should have a boiling temperature above 100 K, but must have a very high cooling capacity for effective cooling of the second gas. In this case, the gases R14 (tetrafluoromethane, CF4) or methane (CH4) are suitable.

The cooling capacity of the "second" gases as a result of high-pressure expansion at the expansion nozzle is all the greater, the cooler the gases are prior to high-pressure expansion at the expansion nozzle. With a cooling apparatus as described above, the first gas is also partly used for pre-cooling the second gas when it mixes with the return flow of the second gas. If a gas combination is selected from the gases proposed above, then the second gas, which serves for cooling the detector, is lowered to a temperature range, which lies far below the inversion temperature and in the range just above its boiling temperature. In this temperature range, the second gas then has, in accordance with its thermodynamic properties, a significantly greater cooling capacity after its expansion than would be achievable by means of a pre- cooling only with the second gas. Thus, an almost complete liquefaction of the second gas can be achieved. With the liquid phase of the second gas, the detector can then be cooled effectively. An especially suitable gas combination is, for example, c.. .:: c. :: . I:: : . ::: .. . 6.... ....

argon as second gas with a boiling temperature of 90 K and tetrafluoromethane as first gas with a boiling temperature of 140 K. In order to obtain cooling temperatures substantially below 90 K, a suitable gas combination involves the use of a neon-argon or neon-nitrogen mixture as second gas for the outer counterflow heat exchanger, and methane with a boiling temperature of 1 13 K (at l bar) as first gas for the inner counterflow heat exchanger.

For missiles with an infrared detector, the use of the gas R14 (tetrafluoromethane) for the inner counterflow heat exchanger and of the gas argon for the outer counterflow heat exchanger has proved to be a particularly suitable gas combination for the cooling apparatus. It is however also conceivable to use the gas methane (CH4) for the inner counterflow heat exchanger and the gases nitrogen or air for the outer counterflow heat exchanger.

When the cooling apparatus is for use in missiles, it is important that the weight that the missile must carry due to the cooling apparatus and the quantities of gas needed for a missile mission is kept to a minimum. On the one hand, this can be achieved in that the flow of the first gas for the inner counterflow heat exchanger can be reduced to a zero quantity depending upon the temperature of the cooled second gas. More specifically, if the gas of the outer counterflow heat exchanger has already been cooled to the desired temperature, then it can be kept at this temperature via its counterflow cooling without the gas of the inner counterflow heat exchanger still being needed for this purpose. It is expedient if, prior to use of the missile or when the missile is actually being developed, it is ascertained what quantities of gas are needed for the inner and outer counterflow heat exchangers so that no more than necessary is carried in the missile. In a practical way, the cooling-gas containers for the two gases required for operating the cooling apparatus are also designed to have correspondingly small dimensions, thus contributing towards a saving of space.

Owing to the possibility that the gas for the inner counterflow heat exchanger can be reduced to a flow quantity of zero or else is ultimately used up after a certain operating time, it is ensured that, in the space downstream of the expansion nozzle of the outer counterflow heat exchanger, there is no excess of liquid phase and thus no problem occurs in respect of temperature stability. Admittedly, the cooling power c: ee: a-. ë:e e se e 7 e e required of the second gas for the outer counterflow heat exchanger is thus greater, but since after the cooling phase for the second gas the cooling power to be dissipated is significantly reduced, the second gas can easily take over the increased cooling burden resulting from removing the cooling power by way of the first gas from the cooling circulation. As already mentioned, this increased cooling burden also specifically prevents the development of an excess of liquid phase of the second gas in the space downstream of the expansion nozzle of the outer counterflow heat exchanger. Thus problems of temperature stability owing to liquid constituents of the second gas in the return flow of the outer counterflow heat exchanger with corresponding jumps in temperature are largely precluded.

In respect of the spaces formed downstream of the expansion nozzles of the two counterflow heat exchangers, consideration must generally be given to suitable dimensioning with regard to their volume, depending on the combination of cooling gases used, their volumes and pressures and action time. These so-called "vapour chambers" must also be geometrically designed so that on the one hand they ensure an optimum cooling capacity, and on the other hand they prevent an excess of liquid phase which adversely affects temperature stability. In particular, the vapour chamber of the inner counterflow heat exchanger for accommodating the gaseous and liquid constituents must be geometrically designed so that on the one hand it ensures an optimum cooling capacity for the detector, and on the other hand it largely prevents a return flow of the liquid phase into the outer counterflow heat exchanger, since there this can lead to extremely variable gas flows, which in turn cause pressure changes in the vapour chamber and thus lead to changes in evaporation point and temperature along the boiling-point curve of the second gas and thus adversely affect the temperature stability of the detector.

The number of openings in the outer casing is usefully produced by a regular perforation. As a result, a particularly good and thorough mixing of the two cooling gases used is ensured, and thus a shorter cooling time and a greater cooling capacity are achieved. The mixed return flow of the first and second gases is directly influenced by the number and size of the holes in the outer casing produced by the perforation process. . ..

A. : hi: .. :: 8.:. . .. ' The geometric arrangement of the perforation is advantageously selected as a function of the combination of cooling gases used and the desired quantity of flow.

A detector is advantageously disposed on the outside of the end plate which closes the outer casing. The end plate is made of a heat-conductive material and thus facilitates the optimum transfer of heat between the liquefied gas, present in the vapour chamber of the outer counterflow heat exchanger, and the detector, without the latter coming into direct contact with the liquefied gas. By way of the thermally conductive heat transfer plate, a homogeneous temperature distribution over the entire detector is achieved. This ensures perfect operation of the detector. This also prevents the detector from suffering any damage due to direct contact with the liquefied gas.

The heat-insulating housing of the cooling apparatus is usefully designed so that the detector is freely exposed towards the front, thus can "look forwards" with a predetermined angle of view. The heat-insulating housing can be a Dewar vessel thermally insulating everything so as thermally to insulate the cooling apparatus from its surroundings.

The heat-insulating housing of the cooling apparatus is advantageously closed at its lower end by a radiation-transparent window for the detector. Thus a cooling apparatus is produced which allows operation of a permanently integrated detector inside the cooling apparatus. By means of the housing with the window, the detector is also protected from damage and external thermal influences.

Furthermore, it is advantageous that the space between the end plate and the window is evacuated. The evacuation produces better thermal insulation from external thermal influences. Since vacuum is a poor heat conductor, the cooling action is concentrated on the detector and is not delivered through this to the environment.

Thus a faultless operation of the detector is ensured which is scarcely affected by thermal noise. The cooling power to be produced by the cooling apparatus during the cooling process thus concentrates specifically on the remaining dissipative paths towards the detector and on the thermal heat capacity of the detector and its fixtures.

ë a: t.. .e e: .e :: 9 it.. ... :: . It is also advantageous that the counterflow heat exchangers comprise tubes provided with ribs and around which plastic threads are drawn. The plastic threads function in precisely the same way as the ribs for the purpose of further improving the heat transfer by gas deflection. The resulting improvement in heat transfer leads to shorter cooling times and thus to quicker operational readiness of the detector. As a result, the cooling capacity can also be improved in that lower temperatures are achieved in respect of the gas of the outer counterflow heat exchanger. Thus the operation of large-surface detectors is also made possible.

A working example of the invention is now explained in greater detail with the aid of a drawing. The illustration in the drawing schematically shows the structure of a cooling apparatus having an inner and an outer counterflow heat exchanger.

The cooling apparatus 10 has an inner counterflow heat exchanger 12 and an outer counterflow heat exchanger 14. The two counterflow heat exchangers 12, 14 are disposed in a heat-insulating housing 16. The inner counterflow heat exchanger 12, the outer counterflow heat exchanger 14 and the housing 16 then lie concentrically relative to one another. The inner counterflow heat exchanger 12 is spatially separated from the outer counterflow heat exchanger 14 by a thin metal outer casing 18.

By way of a gas connection 20, a first high-pressure gas flows through a supply line 22 from a pressure container (not illustrated) into the inner counterflow heat exchanger 12. The first high-pressure gas flows through a tube 26, disposed in the form of a helix around a thermally poorly conductive inner tube 24, to an expansion nozzle 28 located at the end of the tube 26. At the expansion nozzle 28, the first high-pressure gas is Repressurized and thus cooled according to its Joule-Thomson thermal coefficient. Thus the forward flow of the first high-pressure gas is gradually cooled further until, downstream of the expansion nozzle, a liquid consisting of gaseous and liquid constituents is formed from the high-pressure gas. The liquid phase of the cooled high-pressure gas accumulates downstream of the expansion nozzle 28 in a vapour chamber 30 which is formed by a dividing plate 32 located in the outer casing. The liquid, evaporating there at the boiling temperature of the first highpressure gas, then flows together with the non-liquefied gaseous constituent as a ce :: c. 'c: ::e . .. . '. gas in the counterflow around the outer surface of the tube 26 and exits via an outlet 34. As a result, the tube 26 for the forward flow of the first high- pressure gas and thus the high-pressure gas itself is cooled down by the cooling capacity of the first high-pressure gas to close to the boiling point of the first high-pressure gas. For improving the heat exchange between the forward flow of the first high-pressure gas and the return flow of the Repressurized, cooled first high-pressure gas, the tube 26 is provided on the outside with ribs 36 in the form of a helix. In addition, plastic threads 37 are drawn around the outside of the tube for improving the heat transfer or exchange. The plastic threads 37 lead to a turbulent flow in the return flow of the Repressurized, cooled first high-pressure gas, thus increasing the heat exchange with the tube wall and the ribs.

The outer counterflow heat exchanger 14 also comprises a tube 38 which is provided on its outside with ribs 40 and plastic threads 41. Down to the level of the dividing plate 32, the tube 38 of the outer counterflow heat exchanger 14 is thus wound around the outside of the outer casing 18 in the form of a helix. Then the outer counterflow heat exchanger 14 is extended below the dividing plate 32 inside the outer casing 18.

In the same way as with the inner counterflow heat exchanger 12, the region of the outer counterflow heat exchanger 14 which is then located inside the outer casing 18 is wound around a thermally poorly conductive inner tube 42. The tube 38 of the outer counterflow heat exchanger 14 likewise terminates in an expansion nozzle 44.

Downstream of the expansion nozzle, an end plate 46 is disposed which closes the outer casing 18. Thus a vapour chamber 48 is formed for the second high-pressure gas for the outer counterflow heat exchanger 14.

The second high-pressure gas for the outer counterflow heat exchanger 14 flows via a gas connection 49 from a pressure container (not illustrated) via a supply line 50 into the outer counterflow heat exchanger 14. The second high-pressure gas for the outer counterflow heat exchanger 14 then flows through the tube 38 and is Repressurized at the end of the tube 38 at the expansion nozzle 44 and is thus cooled according to its JouleThomson coefficient. The liquid phase of the second high-pressure gas accumulates on the floor of the vapour chamber 48 and there serves by way of its evaporation enthalpy to cool the detector, reverting to the gas phase, i.e. evaporating, :. :.e it. t' te.; at its boiling point. From there, the cooled second high-pressure gas flows along the outer surfaces ofthe tube 38 in the counterflow and exits via the outlet 34.

Owing to the fact that, in the region in which the inner counterflow heat exchanger 12 is surrounded by the outer counterflow heat exchanger 14, the outer casing 18 has a regular perforation 51, some of the cooled first high-pressure gas of the inner counterflow heat exchanger 12 passes through the outer casing 18 into the outer region of the outer counterflow heat exchanger 14. This leads to improved and faster cooling of the forward flow of the second high-pressure gas for the outer counterflow heat exchanger 14 by the counterflowing second high-pressure gas and some of the counterflowing first high-pressure gas of the inner counterflow heat exchanger 12.

Thus, in this region, quantitatively a greater mixture of the two highpressure gases develops in the counterflow of the outer counterflow heat exchanger 14, and this ensures a particularly effective cooling of the forward flow of the second high- pressure gas. From the vapour chamber 48, as from the boiling point of the second high-pressure gas, the forward flow of the second high- pressure gas is pre-cooled by its own expanded gas from the expansion nozzle 44 as far as the portion of the counterflow heat exchanger 14 where the inner counterflow heat exchanger 12 with perforated outer casing begins. As from this region, the outer counterflow heat exchanger 14 is cooled by the mixture of gases consisting of the two expanded, cooled high-pressure gases. As from the dividing plate 32, due to the additional partial flow of the first high-pressure gas from the expansion nozzle 28 at its boiling temperature, the forward flow of the second high- pressure gas in the outer counterflow heat exchanger 14 is particularly intensively and thus quickly cooled by this higher total gas throughput. Thus a very quick cooling of the second high- pressure gas is achieved for cooling the detector.

In respect of the end plate 46, which closes the outer casing 18, it involves a very heat-conductive material. A detector 52 is disposed on the outside of this end plate 46. By way of the heat-conductive material of the end plate 46, the detector 52 is in direct heat exchange with the liquid phase of the second high-pressure gas of the outer counterflow heat exchanger 14 which has accumulated in the vapour chamber 48.

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The heat-insulating housing 16 of the cooling apparatus 10 is closed at the bottom by a radiation-transparent window 54. The window 54 is disposed so that it is located parallel to, and at a certain distance from, the detector 52, enabling the latter to cover the largest possible field of view. The space 56 formed by the heat-insulating housing 16, the window 54 and the end plate 46 is evacuated so as to prevent a heat exchange between detector 52 and the environment.

13.2. 2. 2. '' 2 '"'..

List of Reference Numbers Cooling apparatus 12 Inner counterflow heat exchanger 14 Outer counterflow heat exchanger 16 Heat-insulating housing 18 Outer casing Gas connection 22 Supply line 24 Inner tube 26 Tube 28 Expansion nozzle Vapour chamber 32 Dividing plate 34 Outlet 36 Ribs 37 Plastic threads 38 Tube Ribs 41 Plastic threads 42 Inner tube 44 Expansion nozzle 46 End plate 48 Vapour chamber 49 Gas connection Supply line 51 Perforation 52 Detector 54 Window 56 Space

Claims

ee. :: ce tee ce a:: . c.. : Patent Claims 1. Cooling apparatus

(10) for cooling a detector (52), said apparatus comprising an inner and an outer counterflow heat exchanger (12 and 14 respectively), for a first and a second gas respectively, located in a heat-insulating housing (16), the inner counterflow heat exchanger (12) being disposed inside a part of the length of the outer counterflow heat exchanger (14), and the inner counterflow heat exchanger (12) being spatially separated from the outer counterflow heat exchanger (14) by an outer casing (18), characterized in that a) the outer casing (18) has a dividing plate (32) between an expansion nozzle (28) for the first gas, located at the end of the inner counterflow heat exchanger (12), and the rest of the outer counterflow heat exchanger (14), b) the rest ofthe outer counterflow heat exchanger (14), which extends beyond the inner counterflow heat exchanger (12), is disposed inside the outer casing (1 8), c) downstream of an expansion nozzle (44) for the second gas, located at the end of the outer counterflow heat exchanger (14), the outer casing (18) is closed by an end plate (46), d) in the region in which it is surrounded by the outer counterflow heat exchanger (14), the outer casing (18) has a number of openings.
2. Cooling apparatus (10) according to Claim 1, characterized in that the number of openings is formed by a regular perforation (51).
3. Cooling apparatus (10) according to Claim 1 or 2, characterized in that a detector (52) is disposed on the outside of the end plate (46).
4. Cooling apparatus (10) according to Claim 3, characterized in that the heat-insulating housing (16) is closed at its lower end by means of a radiation-transparent window (54) for the detector (52).

c : : : : c c c c 15.:. .cc c
5. Cooling apparatus (10) according to Claim 4, characterized in that the space (56) between the end plate (46) and the window (54) is evacuated.
6. Cooling apparatus (10) according to one of the preceding claims, characterized in that the counterflow heat exchangers (12, 14) each comprise a tube (26, 38) provided with ribs (36, 40) and having plastic threads (37, 41) drawn around it.

f oolnlg apparatus (10) as substantially described herei ith f